Light-Emitting Diodes (LEDs) have been used in many applications to replace conventional incandescent lamps, fluorescent lamps, Neon tube and fiber optics light sources to reduce electrical and maintenance costs and increase reliability (due to the fact that LEDs consume less electrical energy than many conventional light sources while exhibiting much longer lifetimes). Many designs have been invented for various applications, such as traffic signal lights, channel letter modules, conventional illuminated commercial signs, street signs, etc.
Heat is one of the worst enemies affecting the life of an LED device. Designs to ensure that an LED product doesn't operate at too high of a junction temperature is critical to ensure the reliability of the product. If an LED device operates at too high of a junction temperature, it can often be damaged and its life is reduced. As such, since there is usually an allowable maximum junction temperature determined by LED manufacturers, it is critical that LED devices do not operate above this maximum allowable junction temperature to avoid serious damage.
Heat is usually generated when an electrical current passes through an LED device to produce light. Consequently, the heat will raise the junction temperature of the LED device. When the junction temperature of an LED rises, the quantum efficiency of the device usually decreases. For example, the efficiency of a high power “Blue+phosphors” white LED is about 20% less at 80° C. junction temperature than at 25° C. according to the “Technical Datasheet DS25” of Philips Lumileds Lighting for their high power LED lamps, which is hereby incorporated by reference.
Since heat is continuously generated when an LED device is in operation, heat sinking arrangements are usually included in a luminaire to transfer heat from the LED devices to other parts of the luminaire, and then from these other parts of the luminaire to the surrounding environment such as air. The rate to remove heat, however, is usually slower than the rate heat is generated when the luminaire is just turned on. The temperature of the luminaire and the LED devices thus rise accordingly.
When the luminaire temperature is higher, the temperature gap between the luminaire and the environment is bigger. And when this temperature gap is bigger, the heat dissipation rate to the surrounding environment is higher. An equilibrium temperature is eventually achieved when the heat generated equals the heat dissipated between the luminaire and the surrounding environment. The equilibrium temperature is higher if the heat dissipation capability of the heat sink is lower, and vice versa.
The junction temperature of an LED device will rise accordingly too and will stay at an equilibrium junction temperature. Usually a lower operating current results in a lower equilibrium junction temperature because less heat is generated. And a lower environmental temperature results in easier heat dissipation from the luminaire to the surrounding environment
LED luminaires are operated to produce light flux for lighting up a specific space, wherein a higher operating current results in more flux being produced. However when an LED is driven at a higher current it usually works less efficiently than when it's driven at a lower current. Therefore, a compromised operating current is usually chosen to get as much flux at an acceptable LED junction temperature where the LED efficacy is acceptable.
When a compromised operating current is chosen, it's desirable to operate it at or within a selected range of that current. Beyond the selected range, the luminaire will either produce less than the desired flux or operate at the risk of damaging the life of LED and luminaire.
Some luminaires such as street lights etc. are designed to operate mainly at night when the daylight has disappeared or is disappearing, and when the environmental temperature is lower at that time than at the day time. To turn on or off the light usually depends on a timer, a photo-controller to do it automatically (i.e. by detecting low light levels), or by manual switching to achieve that purpose. When a malfunction happens that these mechanisms fail to turn the light off at dawn, the luminaire could be lit up during the day time when the environmental temperature is hot, especially in the summer time. When this happens, it not only wastes electrical energy and adds unnecessary electricity burden to the power plants, but it can also damage the LED devices since they are working at a much higher environmental temperature. There can also be cases that even at night time, the temperature is high and protection of the LEDs from overheating is also necessary.
Lastly, luminaires are used in different climates and in different surroundings. traditional light sources typically provide a fixed color of light only and usually allow no choices in color once a particular lamp is installed. Changing the hue of lighting is sometimes desirable as the environment changes, for instance when fog comes in etc.
Accordingly, there is a need for a method and apparatus for providing a mechanism which can slow down the operation and the heat generation of the LED product when the temperature of the product is high enough to endanger the life of the LED product, and thus prevent it from a catastrophic breakdown. There is also a need for incorporating a design to change the hue of the lighting if the need arises.